On Tue, Dec 27, 2022 at 7:38 AM Michael S. Tsirkin [off-list ref] wrote:

On Mon, Dec 26, 2022 at 03:49:07PM +0800, Jason Wang wrote:

quoted

This patch introduces a per virtqueue waitqueue to allow driver to
sleep and wait for more used. Two new helpers are introduced to allow
driver to sleep and wake up.

Signed-off-by: Jason Wang <jasowang@redhat.com>
---
Changes since V1:
- check virtqueue_is_broken() as well
- use more_used() instead of virtqueue_get_buf() to allow caller to
  get buffers afterwards
---
 drivers/virtio/virtio_ring.c | 29 +++++++++++++++++++++++++++++
 include/linux/virtio.h       |  3 +++
 2 files changed, 32 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 5cfb2fa8abee..9c83eb945493 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c

@@ -13,6 +13,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/kmsan.h>
 #include <linux/spinlock.h>
+#include <linux/wait.h>
 #include <xen/xen.h>

 #ifdef DEBUG

@@ -60,6 +61,7 @@
                      "%s:"fmt, (_vq)->vq.name, ##args);      \
              /* Pairs with READ_ONCE() in virtqueue_is_broken(). */ \
              WRITE_ONCE((_vq)->broken, true);                       \
+             wake_up_interruptible(&(_vq)->wq);                     \
      } while (0)
 #define START_USE(vq)
 #define END_USE(vq)

@@ -203,6 +205,9 @@ struct vring_virtqueue {
      /* DMA, allocation, and size information */
      bool we_own_ring;

+     /* Wait for buffer to be used */
+     wait_queue_head_t wq;
+
 #ifdef DEBUG
      /* They're supposed to lock for us. */
      unsigned int in_use;

@@ -2024,6 +2029,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
      if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
              vq->weak_barriers = false;

+     init_waitqueue_head(&vq->wq);
+
      err = vring_alloc_state_extra_packed(&vring_packed);
      if (err)
              goto err_state_extra;

@@ -2517,6 +2524,8 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
      if (virtio_has_feature(vdev, VIRTIO_F_ORDER_PLATFORM))
              vq->weak_barriers = false;

+     init_waitqueue_head(&vq->wq);
+
      err = vring_alloc_state_extra_split(vring_split);
      if (err) {
              kfree(vq);

@@ -2654,6 +2663,8 @@ static void vring_free(struct virtqueue *_vq)
 {
      struct vring_virtqueue *vq = to_vvq(_vq);

+     wake_up_interruptible(&vq->wq);
+
      if (vq->we_own_ring) {
              if (vq->packed_ring) {
                      vring_free_queue(vq->vq.vdev,

@@ -2863,4 +2874,22 @@ const struct vring *virtqueue_get_vring(struct virtqueue *vq)
 }
 EXPORT_SYMBOL_GPL(virtqueue_get_vring);

+int virtqueue_wait_for_used(struct virtqueue *_vq)
+{
+     struct vring_virtqueue *vq = to_vvq(_vq);
+
+     /* TODO: Tweak the timeout. */
+     return wait_event_interruptible_timeout(vq->wq,
+            virtqueue_is_broken(_vq) || more_used(vq), HZ);

BTW undocumented that you also make it interruptible.
So if we get an interrupt then this will fail.

Yes, this is sub-optimal.

But device is still going and will later use the buffers.

Same for timeout really.

Avoiding infinite wait/poll is one of the goals, another is to sleep.
If we think the timeout is hard, we can start from the wait.

Thanks

quoted

+}
+EXPORT_SYMBOL_GPL(virtqueue_wait_for_used);
+
+void virtqueue_wake_up(struct virtqueue *_vq)
+{
+     struct vring_virtqueue *vq = to_vvq(_vq);
+
+     wake_up_interruptible(&vq->wq);
+}
+EXPORT_SYMBOL_GPL(virtqueue_wake_up);
+
 MODULE_LICENSE("GPL");

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index dcab9c7e8784..2eb62c774895 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h

@@ -72,6 +72,9 @@ void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
 void *virtqueue_get_buf_ctx(struct virtqueue *vq, unsigned int *len,
                          void **ctx);

+int virtqueue_wait_for_used(struct virtqueue *vq);
+void virtqueue_wake_up(struct virtqueue *vq);
+
 void virtqueue_disable_cb(struct virtqueue *vq);

 bool virtqueue_enable_cb(struct virtqueue *vq);
--

2.25.1